Conventionally, blood pressure has been measured by one of four basic methods: invasive, oscillometric, auscultatory and tonometric. The invasive method, also known as an arterial-line method (or "A-line"), typically involves insertion of a needle or catheter into an artery. A transducer connected by a fluid column to the needle or catheter is used to determine exact arterial pressure. With proper instrumentation, systolic, diastolic, and mean arterial pressures may be determined, and a blood-pressure waveform may be recorded. This invasive method is difficult to set up, is expensive and time consuming, and involves a potential medical risk to the subject or patient (for example, formation of emboli or subsequent infection). Set up of the arterial-line method also poses technical problems. Resonance often occurs and causes significant errors. Also, if a blood clot forms on the end of the needle or catheter, or the end of the needle or catheter is located against an arterial wall, a large error may result. To eliminate or reduce these errors, the apparatus must be checked, flushed, and adjusted frequently. A skilled medical practitioner is required to insert a needle or catheter into the artery, which contributes to the expense of this method. Medical complications are also possible, such as infection, nerve and/or blood vessel damage.
The other three traditional methods of measuring blood pressure arc non-invasive. The oscillometric method measures the amplitude of blood pressure oscillations in an inflated cuff. Typically, the cuff is placed around the left upper arm of the patient and then pressurized to different levels. Mean pressure is determined by sweeping the cuff pressure and determining the cuff pressure at the instant the peak amplitude occurs. Systolic and diastolic pressure is determined by cuff pressure when the pressure oscillation is at some predetermined ratio of peak amplitude.
The auscultatory method also involves inflation of a cuff placed around the left upper arm of the patient. After inflation of the cuff to a point where circulation is stopped, the cuff is permitted to deflate. Systolic pressure is indicated when Korotkoff sounds begin to occur as the cuff is deflated. Diastolic pressure is indicated when the Korotkoff sounds become muffled or disappear.
The fourth method used to determine arterial blood pressure has been tonometry. The tonometric method typically involves a transducer positioned over a superficial artery. The transducer may include an array of pressure-sensitive elements. A hold-down force is applied to the transducer in order to partially flatten the wall of the underlying artery without occluding the artery. Each of the pressure-sensitive elements in the array typically has at least one dimension smaller than the lumen of the underlying artery in which blood pressure is measured. The transducer is positioned such that at least one of the individual pressure sensitive elements is over at least a portion of the underlying artery. The output from one or more of the pressure-sensitive elements is selected for monitoring blood pressure. These tonometric systems either use an upper-arm cuff to calibrate blood-pressure values, or they measure a reference pressure directly from the wrist and correlate this with arterial pressure. However, when a patient moves, recalibration of the tonometric system is often required because the system may experience a change in electrical gains. Because the accuracy of such tonometric systems depends upon the accurate positioning of the individual pressure sensitive element over the underlying artery, placement of the transducer is critical. Consequently, placement of the transducer with these tonometric systems is time-consuming and prone to error. Also, expensive electro-mechanical systems guided by software/hardware computer approaches are often used to assist in maintaining transducer placement.
The oscillometric, auscultatory and tonometric methods measure and detect blood pressure by sensing force or displacement caused by blood pressure pulses within the underlying artery that is compressed or flattened. The blood pressure is sensed by measuring forces exerted by blood pressure pulses in a direction perpendicular to the underlying artery. However, with these methods, the blood pressure pulse also exerts forces parallel to the underlying artery as the blood pressure pulses cross the edges of the sensor which is pressed against the skin overlying the underlying artery of the patient. In particular, with the oscillometric and the auscultatory methods, parallel forces are exerted on the edges or sides of the cuff. With the tonometric method, parallel forces are exerted on the edges of the transducer. These parallel forces exerted upon the sensor by the blood pressure pulses create a pressure gradient across the pressure-sensitive elements. This uneven pressure gradient creates at least two different pressures, one pressure at the edge of the pressure-sensitive element and a second pressure directly beneath the pressure sensitive element. As a result, the oscillometric, auscultatory and tonometric methods can produce inaccurate and inconsistent blood pressure measurements.
Further, the oscillometric and auscultatory methods are directed at determining the systolic, diastolic, and/or mean blood pressure values, but are not suited to providing a calibrated waveform of the arterial pulse pressure.
The traditional systolic-diastolic method for measuring blood pressure provides the physician with very limited clinical information about the patient's vascular health. In contrast, the HDI/PulseWave.TM. DO-2020 system made by Hypertension Diagnostics, Inc., the assignee of the present invention, measures a blood pressure waveform produced by the beating heart that, it is believed, can be analyzed to provide an assessment of arterial elasticity. When the aortic valve closes after the heart has ejected its stroke volume of blood (the blood ejected during each heart beat), the decay or decrease of blood pressure within the arteries prior to the next heart beat forms a pressure curve or waveform which is indicative of arterial elasticity. Subtle changes in arterial elasticity introduce changes in the arterial system that are reflected in the arterial blood pressure waveform and research suggests that these changes in the function and structure of the arterial wall precede the development of coronary artery disease, or the premature stiffening of the small arteries which appears to be an early marker for cardiovascular disease.
Incorporating the physiological phenomena associated with blood pressure waveforms, Drs. Jay N. Cohn and Stanley M. Finkelstein, Professors at the University of Minnesota in Minneapolis, developed in the early 1980's a method for determining a measure of elasticity in both large and small arteries. That technique involved an invasive procedure that placed a catheter connected to a pressure transducer into the patient's artery in order to obtain a blood pressure waveform that could be analyzed using a modified Windkessel model, a well-established electrical analog model which describes the pressure changes during the diastolic phase of the cardiac cycle in the circulatory system.
This "blood pressure waveform" or "pulse contour" analysis method provided an independent assessment of the elasticity or flexibility of the large arteries which expand to briefly store blood ejected by the heart, and of the small arteries (arterioles) which produce oscillations or reflections in response to the blood pressure waveform generated during each heart beat.
By assessing the elasticity of the arterial system, clinical investigators have been able to identify a reduction in arterial elasticity in patients without evidence of traditional risk factors, suggesting the early presence of vascular disease. Furthermore, clinical research data has demonstrated that individuals with heart failure, coronary artery disease, hypertension and diabetes typically exhibit a loss of arterial elasticity. These abnormal blood vessel changes often appear to precede overt signs of cardiovascular disease and the occurrence of a heart attack or stroke by many years. Clinical investigators have also demonstrated an age-related loss of elasticity of both the large and small arteries suggesting that premature stiffening of an individual's arteries is an apparent marker for the early onset of cardiovascular disease.
There is a need to obtain, non-invasively, an accurate, repeatable blood-pressure waveform from the radial artery, in order to avoid the problems associated with invasive procedures such as those described above.
In particular, a sensor approximately 1/2" in diameter and approximately 1/2" in height has been found to provide good results. However, the construction of such a sensor is difficult due to its small size and need to be rigged, sealed, and accurate. Thus there is a need for an improved sensor structure and method.